The invention relates to a tool for RF welding of an insert part with two films to a bag, the arrangement of a central electrode on a tool and the use of a central electrode in an RF welding method.
High-frequency or radio-frequency welding is a welding technique in which radio-frequency energy is introduced into the region which is to be welded. The resulting welded joint is very stable. In particular, it can be watertight.
RF welding methods are often used in order to weld together films to an airtight or watertight bag or tube. In medical product technology this method is used to manufacture pouches into which a tubular opening is inserted. For this purpose, a plastic tube is normally welded to two films and the two films are welded together.
Abele, Plastic jointing methods, Carl Hanser Verlag Munich, 1977, shows multiple methods of this kind. Thus an electrode arrangement is shown in FIG. 243b there, in which the electrode on the lower tool is constructed from a single piece, while the electrode on the upper tool is constructed from three pieces. A central electrode is additionally present. With this tool, in one welding process a welded joint can be created both between tubes to be welded in and two films as well as between the two films themselves.
Another known technique is to perform a weld between a tube and two films in two consecutive process steps: firstly the central electrode has RF energy applied to it, while the integral upper tool and the integral lower tool are each connected to ground. This generates a welded joint along the perimeter of the tube. In the second step a welded joint is created between the films, often along the perimeter of a bag. To achieve this the upper tool has a different polarity applied to it than the lower tool, while the central electrode is connected to neutral. The resulting welded joint between the two films is formed either immediately along the entire perimeter of the bag, or firstly in a small region at the side of the welded-in tube, and then with a bag perimeter welding tool along the perimeter of the bag. In the latter case the two weld regions overlap with respect to the films, in order to create a tight joint.
In general, when using RF welding methods for welding an object into two films it is necessary to distinguish between a small tube that is to be welded in between two films, and longer objects or solid profiled objects that are to be welded in.
When a short tube is to be welded in between two films, a central electrode is often introduced into the tube to form the weld. When a long tube is to be welded in, into which a central electrode cannot be introduced for practical reasons, or when a solid profiled object such as, for example, a cable is to be welded in, tools without central electrodes are used.
In the case of welding methods with a central electrode, the welding process is carried out both via primary fields between the electrodes and via leakage fields, which are formed at the edges of the electrodes. In the case of welding methods without central electrodes on the other hand the welding processes take place with the cable or similar objects for the most part only via leakage fields.
The last cited methods, namely welding in of cables with RF welding methods, is described by Abele in a sequel volume on pages 469, 470.